In order to realize a high-speed and large-capacity optical transmission, a technology for transmitting a plurality of bits within one symbol time has been proposed. Multi-level modulation enables a plurality of bits to be transmitted with one symbol. As an example, QPSK (Quadrature Phase Shift Keying) and m-QAM (Quadrature Amplitude Modulation; m=16, 64, 256, or the like) have been put into practical use. In addition, polarization multiplexing enables signals to be transmitted by using two orthogonal polarization beams.
In recent years, multi-level modulation has been implemented by digital signal processing. As an example, a transmitter includes a digital signal processing circuit and an I/Q (in-phase/Quadrature) modulator. The digital signal processing circuit generates a driving signal from transmission data. The I/Q modulator modulates carrier light by using the driving signal given by the digital signal processing circuit, and generates a modulated optical signal. On the other hand, a receiver includes a front-end circuit and a digital signal processing circuit. The front-end circuit converts the received modulated optical signal into an electric field information signal. The digital signal processing circuit recovers the transmission data according to the electric field information signal.
The I/Q modulator is configured by using, for example, a Mach-Zehnder interferometer illustrated in FIG. 1. In the example illustrated in FIG. 1, an I/Q modulator 1000 includes an I-arm modulator 1001, a Q-arm modulator 1002, and a phase shifter 1003. To the I-arm modulator 1001, an I-arm driving signal and an I-arm bias voltage are given. To the Q-arm modulator 1002, a Q-arm driving signal and a Q-arm bias voltage are given. The I-arm driving signal and the Q-arm driving signal are generated by a digital signal processing circuit. The phase shifter 1003 generates a specified phase difference (for example, π/2) between the I-arm and the Q-arm.
To the I/Q modulator 1000, continuous wave light with a specified wavelength is input. The continuous wave light is split, and is guided to the I-arm modulator 1001 and the Q-arm modulator 1002. The I-arm modulator 1001 modulates the continuous wave light with the I-arm driving signal, and the Q-arm modulator 1002 modulates the continuous wave light with the Q-arm driving signal. The I/Q modulator 1000 combines an optical signal generated by the I-arm modulator 1001 and an optical signal generated by the Q-arm modulator 1002, and outputs a modulated optical signal.
An output optical power of each of the modulators (the I-arm modulator 1001 and the Q-arm modulator 1002) varies periodically with respect to an applied voltage, as illustrated in FIG. 2. In the description below, a point at which the output optical power of the modulator becomes a minimum may be referred to as a “null point (or minimum power transmission point)”.
A driving signal (the I-arm driving signal or the Q-arm driving signal) is given to the modulator such that the center of the driving signal waveform matches the null point, as illustrated in FIG. 2. This operation state is realized by controlling a bias voltage (the I-arm bias voltage or the Q-arm bias voltage) applied to the modulator. In the description below, the center of the driving signal waveform may be referred to as an “operation point”. A method for controlling biases of the respective arms of the I/Q modulator is described, for example, in Japanese Laid-open Patent Publication No. 2000-162563 (Japanese Patent No. 3723358). A method for controlling a bias of a π/2-phase shifter of the I/Q modulator is described, for example, in Japanese Laid-open Patent Publication No. 2007-082094 (Japanese Patent No. 4657860).
When a modulated optical signal is generated by using the I/Q modulator, bias control is performed such that an operation point of a driving signal matches a null point of the I/Q modulator, as described above. However, in the example illustrated in FIG. 2, different modulated optical signals are generated between a case at which the operation point is controlled so as to match a null point A and a case at which the operation point is controlled so as to match a null point B. As an example, in a case in which a BPSK signal is generated by using an NRZ driving signal, a phase of an output optical signal that corresponds to a driving signal is shifted by π between a case at which the operation point is controlled so as to match the null point A and a case at which the operation point is controlled so as to match the minimum point B. Namely, a sign of an output optical signal electric field is inverted, and a logic of each of the bits is inverted in the binary phase modulation.
In recent years, a method to which pre-equalization processing in which a transmission signal is processed so as to improve a signal quality at a receiver by the digital signal processing is applied has been proposed. As an example, a light source frequency deviation of a transmitted optical signal or distortion due to a chromatic dispersion of optical transmission fiber can be compensated for by performing pre-equalization. In order to pre-equalize the light source frequency deviation or a chromatic dispersion in an optical transmission fiber, a phase of an optical signal is shifted. Such a process is realized by performing a specified process on the I-arm driving signal and the Q-arm driving signal in the digital signal processing circuit in the example illustrated in FIG. 1.
However, in a case in which an operation point of a modulator is not appropriately controlled, when a process of adding a phase rotation to an optical signal is assumed, a phase rotation in a reverse direction may be added due to the inappropriate operation point of the modulator. As an example, it is assumed that, when operation points of the I-arm modulator 1001 and the Q-arm modulator 1002 are set so as to match the same null point (for example, the null point A in FIG. 2), a phase of a modulated optical signal generated by the modulators is changed as illustrated in FIG. 3A. In this case, when the operation points of the I-arm modulator 1001 and the Q-arm modulator 1002 are set so as to match null points that are different from each other (for example, the null point A and the null point B), the phase of the modulated optical signal generated by the modulators is changed as illustrated in FIG. 3B. Namely, a phase rotation in a reverse direction is added. When a phase rotation in a direction reverse to the expected direction is added to an optical signal, a transmission quality of the optical signal may be deteriorate, compared with a case in which pre-equalization is not performed.
Further, in the polarization multiplexing transmission, when phase rotations of an X-polarization and a Y-polarization have directions reverse to each other, it may be difficult to split respective polarizations and to compensate for characteristics in a receiver.
This problem is not limited to a case in which the operation points of the I-arm and the Q-arm are not appropriately controlled. Namely, this problem may also arise when a phase of the phase shifter 1003 is controlled so as to be a value other than π/2 (for example, 3π/2).